Cold-curing silicone rubber materials, also referred to as “RTV” (from German “Raumtemperatur-vernetzende”=cross-linking at room temperature) silicone rubber materials, have been known for quite some time as custom-designed materials which have elastic properties. They are used, generally, as sealants or adhesives for glass, porcelain, ceramics, stone, plastics, metals, wood, etc., e.g. in applications like joint filling and sealing compounds in construction and sanitary installations, or as coating agents, e.g. in the electronics industry (Römpp Chemie Lexikon, CD ROM, version 2.0, ed. J. Falbe, Thieme-Verlag, Stuttgart 1999; as well as Ullmanns Enzyklopädie der Technischen Chemie, 4th edition, ed. E. Bartholome, Verlag Chemie, Weinheim 1982, vol. 21, p. 511 et seq.). Use is made especially of single-component RTV silicone rubber materials (RTV-1); These are, for example, plastically mouldable mixtures made of α,ω-dihydroxy-polyorganosiloxanes and appropriate cross-linkers (also referred to as cross-linking agents or hardeners in the art), which are suitable for storing under exclusion of moisture (e.g. within a suitable cartridge) but polymerize under the influence of water or humidity of the air at room temperature. Polymerization, as a rule, takes place by condensation of SiOH groups with appropriate hydrolyzable SiX groups of the cross-linkers.
Depending on the desired chemical and physical properties of the polymerization product, such as, e.g., the desired degree of cross-linking, the solvent resistance, etc., various polyfunctional cross-linkers (hardeners), e.g. tri-functional and/or tetra-functional cross-linkers (hardeners), are usually used together with various polyorganosiloxanes which either are difunctional or carry more functional groups. The most frequently chosen difunctional polyorganosiloxane compounds are α,ω-dihydroxy-polyorganosiloxanes.
Based on the leaving groups (HX) released by the hydrolysis of the cross-linker, a distinction is made with RTV-1 silicone rubber materials between acid systems (HX=acids, such as, e.g., acetic acid, etc.), basic systems (e.g. HX=amines, etc.), and neutral systems (e.g. HX=alcohols, oximes, etc.). RTV-1 silicone rubber materials which are commercially available at this time usually comprise acid systems which hydrolyze with release of acetic acid, or neutral systems which hydrolyze with release of oxime compounds, such as, e.g., butan-2-one oxime (or methyl-ethyl-ketoxime, MEKO, respectively).
Since, when cross-linking, acid RTV-1 rubber materials release acetic acid, which as an aggressive compound has the potential of corroding or decomposing, for example, metals, stone, or mortar, and is additionally associated with a considerable problem of smell, neutrally cross-linking oximosilane cross-linkers, which hydrolyze with release of oxime compounds, such as, e.g., butan-2-one oxime (or methyl-ethyl-ketoxime, MEKO, respectively), are often used for RTV-1 silicone rubber materials at this time.
Yet butan-2-one oxime may cause cancer, as has been discovered recently. Therefore, any further use of compounds releasing butan-2-one oxime, on principle, forbids itself for health reasons. That is why, since 2004, butan-2-one oxime must be labelled by the R phrase (risk phrase) “R40” (“suspected of having cancerogenic effects”). As a consequence, also silicone rubber materials must be thus labelled if they contain free butan-2-one oxime in a concentration above a certain threshold. The labelling requirement particularly includes silicone rubber materials like, for example, the ones contained in sealant cartridges, unless the free content of butan-2-one oxime is less than 1% (cf. “mixing rule” of the preparation directive, Directive 2006/8/EG of the Commission of Jan. 23, 2006, published in the Official Journal of the European Union of Jan. 24, 2006).
Practically all of the conventional cross-linkers mentioned above suffer from another disadvantage, also under health aspects, namely that the compounds released during cross-linking smell awful, sometimes extremely awful, which causes great discomfort, especially, when working with them in closed spaces.
In view of those disadvantages of the conventional cross-linkers, the inventors of the present invention have previously invented a new cross-linker (hardener) on the basis of lactate, in particular on the basis of ethyllactate, which provides a cross-linker, which is non-toxic and does not smell bad, and which does not release aggressive compounds upon cross-linking. This cross-linker is described in detail in EP 2 030 976 A1.
For the desired broad application range of the silicone rubber materials, these should adhere to as many surfaces as possible, such as, e.g., to wood, varnished wood, glazed wood, metals, such as steel, aluminium, powder-coated aluminium, glass, plastics, such as polyvinylchloride (PVC), polyamide, concrete, etc. Moreover, it is important that the silicone rubber materials polymerize as completely as possible in order to prevent a subsequent “bleeding” of incompletely reacted starting materials, etc. In addition, the silicone rubber materials should be stable in storage within an ordinary cartridge, i.e. their properties should not change in dependence of the storage time; preferably, both in a cartridge sealed after filling, and in a cartridge that is already opened and/or partially emptied. Finally, the polymerization product obtained after complete cross-linking (cure) of the silicone rubber material should be transparent or clear, respectively.
Apart from the appropriate selection of the actual components of the polymer, such as cross-linker and polyorganosiloxane, for controlling the polymerization rate and/or polymerization degree, a catalyst is usually added, by which important product properties of the silicone rubber materials are influenced, such as, e.g., the skin formation time (i.e. the time, at which a first complete skin is formed on an applied material), the tack free time (i.e. the time, after which the material no longer exhibits tackiness), the complete cure (i.e. the time, at which the polymerization is completed), etc. For example, the following properties are expected from commercial silicone sealing compounds: a skin formation time of 5 to 15 minutes, a tack free time of 15 to 120 minutes, and a complete cure of maximal 7 days upon application with a height of 10 mm. Further information regarding the skin formation time, tack free time and complete cure can be taken, e.g., from the “Praxishandbuch Dichtstoffe” (3rd edition 1990), which has been published by the Industrieverband Dichtstoffe e.V. (IVD).
Up to now, a metalorganic catalyst was used as a catalyst for silicone rubber materials, such as one ordinarily used for polysiloxanes cross-linking by condensation, in particular a catalyst on the basis of a tin-organic compound, such as, e.g., an alkyl-tin-carboxylate, especially dibutyl-tin-dilaurate and dioctyl-tin-dilaurate. However, such tin-organic compounds exhibit toxicological properties, which have led to the restriction of their use in commercially available products (cf. EU Directive 76/769/EWG of 28.05.2009).
As a replacement, another catalyst, which does not comprise a tin-organic compound, was used for the cross-linking of polysiloxanes in the art so far.
For example, a titanium-based compound can be used as catalyst, as described, e.g., in EP 1 230 298 A1 and EP 2 290 007 A1. However, it is known that a catalyst on the basis of a titanium compound can result in yellowing and/or surface tackiness in the products, and has a slow vulcanizing rate, a wanting storage stability, and an incompatibility with current adhesion promoters (coupling agents) on the basis of aminosilanes.
Moreover, EP 1 230 298 A1 describes a catalyst on the basis of amines and metal salts of tin, zinc, iron, lead, barium, and zirconium, as well as on the basis of chelates of tin. This catalyst results in products having a little yellowing, but is a slow catalyst according to EP 2 290 007 A1.
Therefore, EP 2 290 007 A1 proposes a catalyst on the basis of metal compounds of the I. and II. main and transition groups, i.e. Li, Na, K, Rb, Cs, Be, Mg, Ca, Sr, Ba, Cu, Ag, Au, Zn, Cd, and Hg, in form of pure carboxylates, which result in a product having an acceptable hardening progress upon addition of an acid co-catalyst in form of an organic or inorganic acid. EP 2 290 007 A1 describes the use of this catalyst in silicone rubber materials with cross-linkers (hardeners), which hydrolyze with release of acid (acetic acid) or neutral (alcohols or MEKO, respectively) compounds, respectively, in particular by using a catalyst made from a Li carboxylate or from a Sr carboxylate, respectively.
The use of a catalyst on the basis of Li, Na, K, Mg, Ca, Sr compounds without an additional simultaneous use of an acid co-catalyst is described in EP 2 280 041 A1. Therein, it is further described that in particular the use of Octasoligem lithium or Octasoligem strontium in combination with alkoxy, acetoxy or oximo RTV-1 results in products having desired properties.